"By far the easiest way to rid a tree of ants is to simply submerge the entire thing in water for a day or two. The ants will either drown, or relocate."
Submerging the pot will NOT drown the ants, unless you add a surficant like dish washing soap to the soaking tub. That, unfortunately, could also kill your tree.
Ants, having evolved to live in the ground for perhaps a few hundred million years, have adapated to flooding quite well. Some can survive for weeks underwater in flooded conditions. Others have different strategies. Fire ants form "mats" on the surface of the water, linking their bodies together and using surface tension to hold them up. They can also survive underwater for a time too.
All this means is you will probably have to completely disturb their nest to get rid of them. I'd take a hose and wash a lot (not all, given it's a juniper) of the old soil and ants out of the root mass. I'd also try to run a root hook into the root mass where there are signs of an infestation.
Given that fire ants are nasty little critters with a hefty venomous bite could be painful. Disturbing their nests is like kicking a hornet's nest or poking killer bees with a stick...wear heavy gloves, long pants (cinched at the ankels with rubber bands) and keep the critters on the flushing end of the water...
This may seem extreme, but if you've ever been bitten by fire ants, it's doesn't seem all that unreasonable.